An administrator can bulk assign products to categories (Products, Assign to Category). On this screen, the administrator is given the opportunity to bulk adjust the prices by a fixed amount or by a percentage, adding or subtracting by that amount.
The problem is that the equation used to subtract a given percentage amount from the price is wrong. The equation actually sets that price to that percentage. So, when you want to subtract 10% from the price, you actually get a new price that is 10